mwifiex: support NL80211_HIDDEN_SSID_ZERO_LEN for uAP

mwifiex uAP supports NL80211_HIDDEN_SSID_ZERO_LEN type of hidden
SSID only. NL80211_HIDDEN_SSID_ZERO_CONTENTS is not supported.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>